High reflectance laser resonator cavity

ABSTRACT

A ceramic pump cavity structure for use in solid state lasers wherein the pump cavity comprises sintered alumina having grain sizes of between about 0.3 to 0.5 microns. Sintered alumina pump cavities having grain sizes within this range provide desirable diffuse reflectivity comparable to barium sulfate while at the same time providing a structurally strong cavity which is resistant to cracking and breakage. Combinations of the alumina pump cavity with parasitic light absorbers such as samarium oxide are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to lasers and more particularlyto an improved ceramic structure for use as the pumping cavity in flashlamp pumped, solid state lasers.

2. Description of Related Art

When certain high gain laser materials such as neodymium-doped yttriumaluminum garnet (Nd:YAG) or neodymium-doped gadolinium scandium galliumgarnet (Nd:GSGG) are pumped to a condition of large populationinversion, for example in order to achieve Q-switching, a saturationeffect occurs which limits the laser output energy obtainable regardlessof the level of input pumping energy. This saturation is caused in partby a laser depumping phenomenon resulting from the fact that asignificant amount of fluorescent radiation at the lasing wavelengthescapes laterally from the laser rod into the surrounding pumping cavityand is reflected by the pumping cavity back into the laser rod. Thisreturn radiation stimulates decay from the upper laser transition level,thereby effectively limiting the number of excited ions which can occupythat level and, in turn, limiting the maximum output energy obtainablefrom the laser.

In order to overcome the above problems, solid state lasers aretypically constructed so that both the laser rod and the pumping flashlamp are housing within a samarium-doped glass tube. The samarium-dopedglass provides absorption of radiation at the lasing wavelength (1.06microns) while providing transmission of the pumping radiation.

The samarium-doped glass tube is surrounded with a material having ahigh diffuse reflectivity to achieve uniform illumination of the surfaceof the laser rod by the pumping radiation. The material typically usedto provide the desired high diffuse reflectivity is barium sulfate. Whenbarium sulfate powder is used, it is usually tightly packed between thesamarium-doped tube and an outer concentric aluminum tube.Alternatively, the barium sulfate powder is mixed with conventionalplasticizers and binders and sintered to form a ceramic body used tosurround the doped samarium glass tube. These ceramic bodies aregenerally referred to as pump cavity bodies or simply pump cavities.

U.S. Pat. No. 3,979,696 discloses a Nd:YAG pump cavity in which a fusedquartz or borosilicate glass tube is coated with a polycrystallinepowder, such as samarium oxide. The samarium oxide coated tube providesthe same desirable adsorption as the samarium-doped glass tube, and ismuch less expensive. The contents of this patent are hereby incorporatedby reference.

Although the above-described pump cavities have been found to besuitable for their intended purpose, there has been a continuing need todevelop new materials having high diffuse reflectivity to replace theconventionally used barium sulfate. Although barium sulfate providesexcellent diffuse reflectivity, it is an inherently weak material whichhas a flexural strength of only 600 pounds per square inch (40.83atmospheres). Pump cavity bodies which utilize barium sulfate also tendto be expensive to make, are dusty thereby contaminating the optics andare easily broken or cracked.

Accordingly, there is presently a need to provide new materials toreplace barium sulfate as the diffuse reflector material, particularlyin Nd:YAG laser pump cavities. The new material should provide a highlevel of diffuse reflection which is comparable to barium sulfate whileat the same time being easily molded or otherwise shaped and sintered toform a strong pump cavity body which is dust free and is resistant tobreakage and fracture.

SUMMARY OF THE INVENTION

In accordance with the present invention a new material has beendiscovered for use as the diffuse reflector material in solid statelaser pump cavities. The new material provides diffuse reflection whichis comparable to barium sulfate, while at the same time being much moreresistant to breakage and fracture, and is dust free.

The present invention is based on the discovery that powdered aluminacan be molded and sintered to produce a structurally strong pump cavitybody which provides diffuse reflectivity of the lasing wavelength ofselected laser materials such as Nd:YAG, which is close or equivalent tobarium sulfate.

Powdered alumina has been widely used to form many different types ofceramic articles. Typically, the powdered alumina is mixed with variousplasticizers and/or binders and formed by injection molding or otherconventional techniques into the desired shape. The shaped article isthen sintered at an elevated temperature of between 1600° C. to 1700° C.Sintering or firing at such high temperatures produces alumina ceramicbodies which have relatively large grain sizes, are very dense, andwhich are extremely strong. In accordance with the present invention,alumina powder is sintered at temperatures of 1300° C. to 1425° C. toproduce ceramic bodies having relatively small grain sizes of betweenabout 0.3 to 0.5 microns. It was discovered that these ceramic bodieswhen shaped as laser pump cavities provide diffuse reflectivitycomparable to barium sulfate while at the same time providing increasedstructural strength

As an additional feature of the present invention, the sintered aluminapump cavity is coated with a frit containing samarium oxide to form aglaze to provide for absorption of the lasing wavelength to reducedepumping of the laser due to return radiation.

The above-discussed and many other features and attendant advantages ofthe present invention will be become apparent as the invention becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a partial side sectional view of a laser pumping device inaccordance with the present invention;

FIG. 2 is a sectional view of FIG. 1 taken in the II--II plane; and

FIG. 3 is a sectional view of an alternative preferred exemplaryembodiment in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary laser pumping device in accordance with the presentinvention is shown generally at 10 in FIGS. 1 and 2. The device 10includes a rod 12 of laser material and a pumping flash lamp 14. Thelaser material comprises Nd:YAG or Nd:GSGG or other lasing materialhaving pump bands within the range of 0.4 to 2.0 micrometers. The rod 12and flash lamp 14 are mounted inside of sintered alumina body 16. Thealumina body 16 includes an interior surface 18 which defines the pumpcavity 20 in which the laser rod 12 and pump flash lamp 14 are housed.Preferably, the alumina body 16 will include a coating of samarium oxideglaze 22 in order to provide desired absorption of parasitic wavelengthsaround 1.06 microns.

The flash lamp 14 emits pumping radiation which excites the material ofthe rod 12 to a condition in which population inversion is establishedbetween a pair of energy levels of the laser material. As a result,laser radiation is emitted from the ends of laser rod 12 at a wavelengthcorresponding to the energy difference between the pair of energy levelsin question. As an example, when the laser rod 12 is made of Nd:YAG orNd:GSGG, the desired pumping radiation may be provided by a xenon flashlamp 1 4, creating laser emission at wavelengths of approximately 1.06microns due to laser transitions between the ⁴ F_(3/2) and ⁴ F_(11/2)energy states of neodymium.

A pair of aligned reflectors 24 and 26 are disposed adjacent oppositeends of laser rod 12 to provide an optical resonator for reflectingemitted laser radiation back into the rod 12 in regenerative fashion.Q-switching operation may be achieved by locating conventionalQ-switching elements between an end of the rod 12 and one of thereflectors, such as reflector 24. The Q-switching elements may includean electro-optic device 28, such as a Kerr cell or a Pockels cell and apolarizer 30, such as a Nicol polarizer or a Glan-Thomson prism. Theabove-described laser elements are all conventional and well-knownexcept for the ceramic structure in accordance with the presentinvention which is composed of the sintered alumina body 16 and thesamarium glaze 22. The details of this ceramic structure will bedescribed below, with it being understood that the ceramic structure maybe utilized in a wide variety of laser devices other than the specificexemplary embodiment described above.

The sintered alumina body 16 can be made according to any number ofconventional sintering procedures so long as the final ceramic body 16has grain sizes of between about 0.3 to 0.5 microns. The optimum grainsize is a function of the wavelength of the radiation being pumped, andcan be adjusted accordingly to obtain the optimum diffuse reflectance,and strength. For a pump band of 0.4 to 2 micrometers, grain sizesbetween about 0.3 to 0.5 microns are optimum. Preferably, the requiredgrain size is achieved by mixing alumina powder with appropriate bindersand at least one plasticizer to form a thermosetting mixture which canbe molded according to conventional procedures to form a "green" bodyhaving the desired tubular shapes shown in FIGS. 1 and 2. Standardinjection molding machines can be used as well as conventional moldingtechniques. It is important that the initial alumina powder haveparticle sizes below about 0.6 microns. Preferably, the alumina powdershould have particle sizes in the range of from about 0.28 to 0.50microns. The alumina powder can be any of the commercially availablepowders which have particle sizes in the desired range and which arepreferably at least 99.9 percent by weight pure. Suitable commerciallyavailable powders include those available from the Aluminum Company ofAmerica (ALCOA), such as grade A-16SG. Grade A-16SG is a preferredmaterial. Other commercially available alumina powders can also be usedsuch as ALUMALUX 39 which is also available from ALCOA.

The alumina powder is preferably mixed with two or more binders and atleast one plasticizer. The binders are preferably thermosetting below200° C. Sufficient binders and plasticizers are added so that themixture is 70 percent to 90 percent solids and very fluid (lowviscosity) at a temperature of about 200° C. to 350° C. As isconventional, the hot alumina slurry is injected into an appropriatelyshaped die at this temperature and at pressures ranging from 30 psi(2.04 atmospheres) to 15,000 psi (1020.7 atmospheres). The fluid orslurry is allowed to cool within the die and becomes a solid attemperatures ranging from 100° C to 200° C. The formed part is thenejected from the die.

The above-described basic alumina molding procedure allows reproductionof parts with extremely complex geometry if desired. Standard injectionmolding machines can be used if desired. Suitable binders include thoseconventional binders used in the molding of alumina such as polyethylenebinders and polyvinyl alcohol binders. Plasticizers which may be usedalso include any of the conventional plasticizers such as stearic acid.The amount and number of binders and plasticizers utilized may be variedso long as the injection mixture is from 70 percent to 90 percent byweight solids and is very fluid (i.e. low viscosity) at temperatures ofbetween 200° C. to 350° C. Further, the plasticizers and binders must bepresent in sufficient quantities to provide as near to perfectreplication of the die cavity as possible during molding and also holdthe molded alumina article together during sintering.

Typically, the molded alumina body is subjected to either liquid phaseor gas phase extraction prior to sintering in order to remove theplasticizers. Only one binder is allowed to remain in the molded aluminabody in order to provide the necessary adhesion and structural integrityduring the sintering or firing process. The sintering or firing of themolded alumina body is preferably conducted at atmospheric pressure andin an air atmosphere. Other firing pressures and atmospheres arepossible as is well known for conventional techniques of firing alumina.

In accordance with the present invention, the firing temperature for thealumina body is between 1300° C. and 1425° C. and preferably about 1400°C. The duration of firing will be from one to four hours with preferredfiring or sintering times being between about 0.8 to 1.2 hours. Thesintering temperature and time is varied to provide a final sinteredalumina body having grain sizes of between about 0.3 to 0.5 microns,which provides maximum diffuse reflectance. Grain sizes of about 0.45microns are preferred. It is also preferred that the firing temperaturebe kept close to 1400° C. in order to reduce the time necessary forformation of the ceramic body. The desired grain size range can beverified by scanning electron microscopy or other suitable technique.

The packing density for the final sintered alumina body should bebetween about 70 to 87 percent. The preferred packing density is about85 percent.

Grain growth inhibitors may be used during the sintering process. Manyof the commercially available alumina powders include conventional graingrowth inhibitors such as magnesium oxide. The amount of grain growthinhibitor present in the alumina is preferably less than 2,000 parts permillion (ppm). Alumina powder such as ALCOA Grade 16G has about 200 ppmmagnesium oxide and can be suitably used in forming alumina bodies inaccordance with the present invention. The amount of grain growthinhibitor is not particularly critical so long as the firingtemperatures and times are adjusted accordingly so that the final grainsizes of the sintered alumina body is between about 0.3 to 0.5 microns.

Other conventional procedures may be utilized to form the alumina pumpcavities so long as the final product includes grain sizes as set forthabove. For example, the alumina can be cold pressed at high pressures(i.e. around 5,000 psi) to form an alumina ceramic body having thedesired grain size, packing density, diffuse reflectivity and strength.Typically, cold press procedures involve mixing the alumina with asuitable binder such as a three percent solution of paraffin wax intrichloroethane. The cold press formation of alumina bodies isconventional and any of the well-known techniques may be utilizedprovided that the desired criteria with regard to grain size and packingdensity are achieved as described above.

As previously mentioned, it is preferred that a glaze of samarium oxide18 be applied to the interior surface 22 of the alumina pump cavity 16.The samarium oxide glaze is preferably from 1 thousandths of an inch(0.025 millimeter) to 5 thousandths of an inch (0.127 millimeter) thick.The samarium oxide glaze is preferably applied in the form of a fritwhich is then fired at temperatures of about 1300° C. to form thetransparent glass or glaze layer. The composition of the frit (powderedglass) which has provided efficient absorption of 1.06 microns radiationis provided in the following table:

    ______________________________________                                        Oxide       Weight % (w/o)                                                    ______________________________________                                        Li.sub.2 O  4.15                                                              Al.sub.2 O.sub.3                                                                          14.16                                                             SiO.sub.2   33.38                                                             B.sub.2 O.sub.3                                                                           9.68                                                              SrO         14.40                                                             Sm.sub.2 O.sub.3                                                                          24.22                                                             ______________________________________                                    

The frit is "painted" onto the Al₂ O₃ pump cavity as a paint or paste.The paste uses a fugitive binder like NICROBRAZ cement (8 w/opolyethylmethacrylate in 1,1,1-trichloroethane). The frit is melted ontothe Al₂ O₃ structure at 1300° C., forming a smooth transparent glaze.

Techniques for applying a layer of an oxide glaze or glass frit toceramic materials are well known. Preferably, the samarium frit is mixedwith any of the commercially available cements to form the paste orsuspension which is applied by spraying, or other suitable applicationto the entire surface of the alumina body. Typical commercial cementsinclude an 8 percent by weight solution of polyethyl methacrylate in1'1'1'-trichloroethylene. It is preferred that the entire sinteredaluminum oxide structure be covered with glaze because this increasesthe strength of the material and in addition keeps the body clean duringhandling and use.

An alternative embodiment of the present invention is shown in FIG. 3.In this embodiment, a samarium-doped glass lining 32 is provided on theinterior surface 34 of the alumina pump cavity 36. Samarium-doped glassis widely used in Nd:YAG lasers and can also be utilized in conjunctionwith the sintered alumina pump cavity body of the present invention. Asamarium oxide coated glass tube according to U.S. Pat. No. 3,979,696may also be used.

The improved reflectance achieved by a structure formed in accordancewith the present invention is shown in Table I. The reflectance valuesin Table I are direct readings from an integrating spherespectrophotometer with a 1.25% correction for diffuse reflectance. Asindicated in item 4 of Table I, a structure in accordance with thepresent invention formed from ALCOA 16SG grade aluminum oxide, byinjection molding and sintered at 1400° C. for 1 hour to produce anaverage particle size of about 0.45 micrometers, had a reflectance of96.8%. This value compares very favorably with the 97% reflectanceobtained from a structure of barium sulfate (item 1 of Table I), whichis the accepted standard for reflectance. By contrast, aluminum oxidestructures formed by prior art methods had reflectances of 94.0% and94.3%, as shown in items 2 and 3 of Table I. Furthermore, a structureformed in accordance with the present invention by cold pressing andthen sintering was found to have a flexural strength of approximately23,000 psi (1565.2 atmospheres). The latter value is 21 times that ofsimilar BaSO₄ structures, which has a flexural strength of about 600 psi(40.8 atmospheres).

                  TABLE I                                                         ______________________________________                                        REFLECTANCE DATA                                                                                          Diffuse                                           Item  Sample Description    Reflectance*                                      ______________________________________                                        1     Reference Standard - BaSO.sub.4                                               Eastman - 6091                                                                Average particle size =                                                       1.5 ± 0.5 microns                                                          Injection molded                                                              Sintered at 1050° C.                                                                         97%                                                     Sintered at 900° C.                                                                          98.8%                                             2     Aluminum oxide, 99.7% pure                                                                          94.3%                                                   (A973 Grade from R&W products)                                                Average particle size =                                                       4-5 microns                                                                   Cold pressed                                                                  Sintered at 1700-1710° C.                                        3     Aluminum oxide, 99.7% pure                                                                          94.0%                                                   (Ceralox from Parmatech Inc.)                                                 Average particle size = 4-5 microns                                           Cold pressed                                                                  Sintered at 1700-1710° C.                                        4     Present invention     96.8%                                                   Aluminum oxide (ALCOA 16SG grade)                                             Average particle size = 0.45 microns                                          Injection molded                                                              Sintered at 1400° C. for 1 hour                                  ______________________________________                                         *At 700 nanometers                                                       

EXAMPLE 1

Alcoa grade A-16SG aluminum oxide (Al₂ O₃), average particle size 0.3 to0.5 microns, was made into a slurry with 1,1,1,trichloroethane (solvent)containing 0.1 gram of paraffin wax per cubic centimeter of solvent. Thevolume of solvent was calculated to provide 3 weight percent (w/o) ofparaffin wax in the Al₂ O₃ when dry or free of solvent. The slurry waswet ball milled in the solvent until thoroughly mixed. The solvent wasallowed to evaporate and the Al₂ O₃ was cold pressed at 5,000 psi (340.2atmospheres) pressure into disks measuring 1.13 inches in diameter and0.140 inch thick. The disks were calcined or heat treated to 1400° C.for 60 minutes. The final density of parts were 84% of theoretical. Thediffuse reflectance of the disks were measured in an integrating spherespectrophotometer at 700 nanometer wavelength. The diffuse reflectancewas found to be 101% of the Eastman Kodak standard BaSO₄ (Catalogue#6091, batch #502-1).

EXAMPLE 2

Alcoa grade A-16SG Al₂ O₃ (0.3 to 0.5 microns particles) was mixed withan appropriate amount of magnesium oxide (MgO) (grain growth inhibitor)to provide an MgO content of 700 ppm. The Al₂ O₃ . 0.07 w/o MgO mixturewas saturated with 3 w/o paraffin wax as in Example 1 above. The mix wasplaced in a rubber boot and cold isostatically pressed at 30,000 psi(2041.5 atmospheres). Parts were machined to the geometry shown inFIG. 1. The parts were heat treated at 1425° C. for 60 minutes. Thefinal density was 78% of theoretical. Diffuse reflectance measurementswere made as in Example 1 above and the reflectance was 100% of theBaSO₄ standard.

EXAMPLE 3

Alcoa grade--ALUMULUX-39 (a high purity grade Al₂ O₃ --99.9%) averageparticle size 0.3 to 0.5 microns was mixed with the following materialsshown as follows:

    ______________________________________                                                           Weight Percent                                             ______________________________________                                        A        ALUMULUX 39 - Al.sub.2 O.sub.3                                                                85.0                                                 B        Polystyrene     6.0                                                  C        Polyethylene    0.75                                                 D        Wesson Oil      6.0                                                  E        Stearic Acid    2.25                                                 ______________________________________                                    

The above mix was injection molded at 5,000 psi (340.2 atmospheres) and170° C. into a die having the geometry of FIG. 2. The plasticizers wereremoved with ethyl alcohol and the polystyrene was removed with1,1,1,trichloroethane.

The parts were heat treated for one hour at 1400° C. Final density was78% of theoretical. Reflectance measurements were made as in Example 1and found to be 100.2% of the BaSO₄ standard.

EXAMPLE 4

Aluminum oxide disks (0.9 inch (2.29 cm) in diameter×0.12 inch (0.30 cm)thick) fabricated by the processes described in Example 1 were selectedfor evaluating the samarium glaze. Calculations were made based on thedensity of the samarium glaze (3.2 grams per cubic centimeter) and thesurface area of the Al₂ O₃ disks to provide the weight of samarium oxidefrit (powdered glass) to yield samarium glaze thicknesses on the Al₂ O₃disks of 0.001 inch (0.025 millimeter), 0.003 inch (0.076 millimeter),0.005 inch (0.127 millimeter), 0.007 inch (0.178 millimeter) and 0.009inch (0.229 millimeter). The calculated weight of samarium frit wasplaced in the center of each disk. The Al₂ O₃ disks with frit wereplaced in a furnace and heated to 1300° C. for 60 minute time periods.The frit melted, flowed over the top surface of the disks providing auniform thickness glaze which was very smooth and transparent to thevisible light spectrum. Subsequent tests of this glaze indicated anextremely strong bond to the Al₂ O₃ substrate by virtue of the nominal15% porosity of the substrate. Furthermore the very smooth nature of theglaze provided a surface which did not collect dirt or debris and couldbe wiped clean just like a glass window. Absorption measurements weremade on all five disks (0.001 to 0.009 inch or 0.0254 to 0.229millimeter thicknesses) in an integrating sphere spectrophotometer from220 nanometers to 2400 nanometers. Absorption was found at precisely1.06 microns. There was zero absorption for a BaSO₄ standard and anuncoated (noglaze) Al₂ O₃ disk. The absorption for the 0.001 inch thickglaze was significant and progressed in textbook fashion as thethickness increased from 0.001 inch (0.025 millimeter), 0.003 inch(0.076 millimeter), 0.005 inch (0.127 millimeter), 0.007 inch (0.178millimeter) and 0.009 inch (0.229 millimeter).

The five disks exhibited very low absorption from 520 to 800 nanometers.The glaze did absorb in the ultraviolet regions (350 nm) which isdesirable in certain laser systems. Accordingly, the glaze performsseveral desired functions as just described.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but is only limited by the followingclaims.

What is claimed is:
 1. A ceramic structure for use as a laser pumpcavity to surround a rod of laser material and a light source emittingradiation within a range of 0.4 to 2.0 microns, and wherein laserradiation at a lasing wavelength is produced by said laser material uponexposure to said light source, said ceramic structure comprising aceramic body having an interior surface defining said pump cavitywherein said ceramic body comprises sintered alumina having grain sizesof between about 0.3 to 0.5 microns and a packing density between about70 to 87 percent such that said grain size and said packing densityprovide optimum diffuse reflectance of said radiation within said pumpcavity.
 2. A ceramic structure according to claim 1 wherein said ceramicbody is adapted for use in a Nd:YAG laser.
 3. A ceramic structureaccording to claim 2 wherein said grain sizes are approximately 0.45microns.
 4. A ceramic structure according to claim 1 wherein saidceramic body is adapted for use in a Nd-doped gadolinium scandiumgallium garnet laser.
 5. A ceramic structure according to claim 1 whichfurther includes on said interior surface a coating of a glaze whichabsorbs radiation at said lasing wavelength.
 6. A ceramic structureaccording to claim 5 wherein said glaze comprises samarium oxide.
 7. Aceramic structure according to claim 1 wherein said packing density isabout 85 percent.
 8. A laser pumping device comprising:a rod of lasermaterial, a light source optically coupled to said rod for emittingpumping radiation within the range of 0.4 to 2.0 microns to establishpopulation inversion in said laser material between a pair of energylevels having an energy difference therebetween such that said rod emitslaser radiation at a lasing wavelength corresponding to said energydifference between said energy levels; and a ceramic structuresurrounding said rod and said light source, said ceramic structurecomprising a ceramic body comprising sintered alumina having grain sizesof between about 0.3 to 0.5 microns and a packing density between about70 to 87 percent to thereby optimize the diffuse reflectance of saidpumping radiation.
 9. A laser pumping device according to claim 8wherein said laser material is selected from Nd:YAG and Nd:GSGG.
 10. Alaser pumping device according to claim 8 wherein said grain sizes areapproximately 0.45 microns.
 11. A laser pumping device according toclaim 8 wherein said ceramic body includes an interior surface defininga pump cavity surrounding said rod and said light source, said ceramicstructure further including a coating of a glaze containing samariumoxide on said interior surface.
 12. A ceramic structure for use as alaser pump cavity to surround a rod of laser material and a light sourceemitting radiation within a range of 0.4 to 2.0 microns and whereinlaser radiation at a lasing wavelength is produced by said lasermaterial upon exposure to said light source, the improvement comprisingsaid structure being made by sintering alumina powder at a temperatureof about 1400° C for 0.8 to 4 hours to form a sintered alumina ceramicbody having grain sizes of between about 0.3 to 0.5 microns and apacking density between about 70 to 87 percent, thereby producing adiffuse reflectance within said pump cavity and a packing densitybetween about 70 to 87 percent.
 13. A method for optimizing reflectanceof radiation within a laser pump cavity, said radiation having a rangeof 0.4 to 2.0 microns comprising placing said light source and saidlaser rod within said pump cavity comprising sintered alumina havinggrain sizes of between 0.3 to 0.5 microns and a packing density betweenabout 70 to 87 percent whereby said alumina optimizes said reflectanceof said radiation.